Open a row downwards, from whichever end was pressed

Tapping a group's heading used to send that heading up off the top of the
screen and fill the space above it, so the calls appeared on the far side
of the control that produced them. The transcript is laid out from the
bottom, so every row's bottom edge is what the list holds still and all
growth goes upward.

The rule now is that the end the reader pressed is the end that must not
move. A heading anchors the top, so the row opens downwards under it; the
bar at the foot of an open group anchors the bottom, so shutting it from
there leaves what follows the group where it is -- which is what already
happened, but by accident of the layout rather than on purpose, and would
have been lost the moment anything else changed.

Bottom is the list's own behaviour and costs nothing. Top is measured
rather than calculated: only the layout knows how tall an open group is,
so `toggleAnchored` reads where the top edge was, lets the change land,
and scrolls by however far it moved.

Applied to every row that opens, not just groups -- a lone tool call and a
peer message are the same gesture, and one of them opening the other way
would be the odder for it.

app/androidApp/src/main/kotlin/com/example/aiapp/ToolRows.kt

@@ -99,6 +99,20 @@ fun groupToolRuns(items: List<TranscriptItem>): List<TranscriptRow> {
     return rows
 }
 
+/**
+ * Which end of a row a reader acted on, and therefore which end must not move.
+ *
+ * A row has two controls at opposite ends -- the heading that opens it and the bar that shuts it
+ * again -- and the reader's finger is on one of them. Whichever it is has to stay where it is while
+ * the row changes size, or the thing they just pressed slides out from under them. The list anchors
+ * every row's bottom edge by default (see the transcript's `reverseLayout`), so [Bottom] is what
+ * happens on its own and [Top] is what has to be arranged.
+ */
+enum class RowEdge {
+    Top,
+    Bottom,
+}
+
 /**
  * Several calls under one heading, closed until somebody asks.
  *
@@ -113,14 +127,15 @@ fun groupToolRuns(items: List<TranscriptItem>): List<TranscriptRow> {
 fun ToolGroup(
     group: TranscriptRow.Tools,
     expanded: Boolean,
-    onToggle: () -> Unit,
+    /** Told which end was pressed, because this row has a control at each -- see [RowEdge]. */
+    onToggle: (RowEdge) -> Unit,
     isToolExpanded: (String) -> Boolean,
     onToolToggle: (String) -> Unit,
     onAnswer: (questionId: String, answers: List<String>) -> Unit,
     image: @Composable (String) -> Unit,
 ) {
     if (!expanded) {
-        Card(Modifier.fillMaxWidth().clickable(onClick = onToggle)) {
+        Card(Modifier.fillMaxWidth().clickable { onToggle(RowEdge.Top) }) {
             Text(
                 "Called ${group.calls.size} tools",
                 style = MaterialTheme.typography.titleSmall,
@@ -133,7 +148,7 @@ fun ToolGroup(
         Text(
             "Called ${group.calls.size} tools",
             style = MaterialTheme.typography.titleSmall,
-            modifier = Modifier.fillMaxWidth().clickable(onClick = onToggle).padding(12.dp),
+            modifier = Modifier.fillMaxWidth().clickable { onToggle(RowEdge.Top) }.padding(12.dp),
         )
         group.calls.forEach { call ->
             ToolCard(
@@ -144,7 +159,9 @@ fun ToolGroup(
                 image = image,
             )
         }
-        CollapseBar(onToggle)
+        // Shutting it from here anchors the other end: the reader is at the bottom of a long
+        // group, and what they are looking at is what follows it.
+        CollapseBar { onToggle(RowEdge.Bottom) }
     }
 }